Key Difference Between Echinoderms And Chordates Symmetry

Echinoderms and chordates, both belonging to the deuterostome group, represent fascinating branches of the animal kingdom. While sharing a common ancestor and certain developmental similarities, a crucial feature sets them apart: their body symmetry. Echinoderms exhibit radial symmetry as adults, whereas chordates possess bilateral symmetry. Understanding this fundamental difference is key to appreciating the unique evolutionary paths of these two groups.

Radial Symmetry in Adult Echinoderms

Radial symmetry, the defining characteristic of adult echinoderms, is a body plan where body parts are arranged around a central axis. Imagine a starfish: its arms radiate outwards from a central disc. This arrangement allows the animal to interact with its environment from all directions, a distinct advantage for a sessile or slow-moving marine organism.

Echinoderms, including starfish, sea urchins, sea cucumbers, and brittle stars, exemplify this body plan. Their radial symmetry is not merely superficial; it extends to their internal anatomy as well. The water vascular system, a hydraulic network unique to echinoderms, operates on a radial plan, facilitating locomotion, feeding, respiration, and excretion. This intricate system, with its network of canals and tube feet, underscores the functional significance of radial symmetry in echinoderms. For instance, a starfish uses its tube feet, powered by the water vascular system, to grip surfaces, move across the seafloor, and even pry open the shells of its prey. Sea urchins, with their spherical bodies and radial arrangement of spines, also rely on this symmetry for defense and movement.

Furthermore, the pentaradial symmetry observed in most echinoderms (fivefold symmetry) is a derived characteristic, meaning it evolved later in their lineage. This pentaradial plan is not present in their larval stages, which exhibit bilateral symmetry, hinting at their evolutionary history and relationship to bilaterally symmetrical ancestors. The transition from bilateral symmetry in larvae to radial symmetry in adults is a remarkable developmental transformation, reflecting the adaptation of echinoderms to their benthic marine lifestyles. The evolution of radial symmetry in echinoderms is closely tied to their sedentary or slow-moving existence on the seafloor. This lifestyle favors an arrangement where sensory and feeding structures are distributed around the body, allowing the animal to respond to stimuli and acquire food from any direction. In contrast, bilaterally symmetrical animals, with their concentration of sensory organs and nervous tissue at the anterior end, are better suited for directional movement and active predation.

Bilateral Symmetry in Chordates

In stark contrast to the radial symmetry of adult echinoderms, chordates, including vertebrates like humans, fish, birds, and reptiles, exhibit bilateral symmetry. This body plan is characterized by a distinct left and right side, a dorsal (back) and ventral (belly) surface, and an anterior (head) and posterior (tail) end. Bilateral symmetry is a hallmark of actively moving animals, facilitating streamlined movement and cephalization, the concentration of sensory organs and nervous tissue at the anterior end.

This body plan is not just about external appearance; it deeply influences internal organization. The digestive system, circulatory system, and nervous system in chordates are all organized along a bilateral axis. This arrangement allows for efficient processing of information, coordinated movement, and complex behaviors. The evolution of bilateral symmetry in chordates is closely linked to their active lifestyles and predatory habits. The concentration of sensory organs at the anterior end allows them to better detect prey and navigate their environment. The streamlined body shape reduces drag, enabling efficient movement through water or air.

The development of bilateral symmetry is controlled by complex genetic and developmental processes. During embryogenesis, specific genes are activated in different regions of the developing embryo, establishing the body axes and laying the foundation for bilateral organization. The Hox genes, a family of highly conserved genes, play a crucial role in this process, determining the identity of different body segments along the anterior-posterior axis. The evolution of bilateral symmetry was a major evolutionary innovation, paving the way for the diversification of animal life and the emergence of complex body plans. It allowed for the development of specialized appendages, such as limbs and fins, which further enhanced locomotion and manipulation of the environment. The bilateral body plan is not unique to chordates; it is also found in many other animal groups, including arthropods, mollusks, and annelids. However, chordates have further elaborated on this basic plan, developing unique features such as a notochord, a dorsal hollow nerve cord, and pharyngeal slits.

The Evolutionary Significance of Symmetry Differences

The difference in symmetry between echinoderms and chordates reflects their divergent evolutionary paths and adaptations to different lifestyles. While both groups belong to the deuterostome lineage, their adult body plans showcase distinct strategies for survival and reproduction. The radial symmetry of echinoderms is an adaptation to their sessile or slow-moving marine existence, while the bilateral symmetry of chordates is a key feature for active movement and predation.

This divergence in symmetry is not just a morphological difference; it has profound implications for their physiology, behavior, and ecological roles. Echinoderms, with their radial symmetry, are well-suited for filter-feeding, scavenging, or grazing on the seafloor. Their ability to sense and respond to stimuli from all directions allows them to efficiently capture food and avoid predators. Chordates, with their bilateral symmetry, are typically more active and mobile, often occupying higher trophic levels in the food web. Their cephalization and specialized sensory organs enable them to hunt prey, navigate complex environments, and engage in social interactions.

Understanding the evolutionary significance of symmetry differences between echinoderms and chordates provides valuable insights into the diversity of life on Earth. It highlights how different body plans can arise through adaptation to different ecological niches and how these adaptations can shape the evolution of entire lineages. The study of echinoderm and chordate symmetry also offers a window into the developmental processes that underlie body plan formation. By comparing the genetic and molecular mechanisms that control symmetry development in these two groups, scientists can gain a deeper understanding of the evolution of animal form.

In conclusion, the fundamental difference in body symmetry – radial in adult echinoderms and bilateral in chordates – is a key feature that separates these two deuterostome groups. This difference reflects their unique evolutionary histories and adaptations to distinct lifestyles. While echinoderms have embraced a radial plan suited for their sessile or slow-moving existence, chordates have evolved a bilateral plan that facilitates active movement and complex behaviors. Studying these contrasting body plans provides valuable insights into the diversity of animal life and the evolutionary processes that have shaped it.